Two proteins are described which bind androgen with high affinity, low capacity and which demonstrate androgen specificity. One of these (androgen binding protein, ABP) is produced in the Sertoli cell of the testis, secreted into lumina of the seminiferous tubules, and transported via the efferent ducts to the epididymis. We have purified ABP to homogeneity raised antibodies and have prepared a radioimmunoassay for its detection. Another binding protein, androgen cytoplasmic receptor (CR) has been reported in a number of androgen target organs, including the Sertoli cell of the testis and seminal vesicle. Its physiochemical properties are quite different from ABP, but its affinity for dihydrotestosterone is similar to that of ABP. We have recently purified the androegen receptor to homogenity. A basic knowledge of the active binding sites of these two proteins is important in understanding the dynamic equilibrium of hormones. In this proposal we plan to fully characterize each protein with respect to its physicochemical properties, and raise monospecific antibodies to the receptor protein. The binding properties of CR, and ABP will be assessed, including steroid specificity, kinetics and thermodynamics of binding and peptide mapping the binding sites. Monospecific antibodies to CR and ABP will be used to study the hormonal regulation of the synthesis of each protein and secretion of ABP.